The present invention refers to a pipe-in-pipe assembly that is adapted for transport of fluids. The pipe assembly is specifically useful in a system for long distance transport of hydrocarbon fluid produced in a subsea hydrocarbon production setup, wherein the system comprises flow assurance through wax and hydrate control by cooling and heating. In accordance herewith, the present invention refers also to a flow assurance system comprising the pipe assembly of the present invention.
Long distance transport of hydrocarbons through a pipeline in seawater from a subsea well to a surface or land-based production facility usually involves the task of controlling and removing deposits of wax/paraffin that is dissolved in the hot fluid and precipitate on the inner wall of the pipeline which is cooled by the surrounding sea. The deposits may grow rapidly to cause a reduction of the pipeline cross-section, and may ultimately result in a complete blockage of the pipeline if countermeasures are not applied.
A commonly used method to avoid clogging of the pipeline is to remove wax deposits from the pipeline wall by forcing a solid plug or “pig” through the pipeline, this way mechanically dislodging the wax. The wax is returned as solid particles to the fluid stream for further transport at a temperature below the wax formation temperature.
Another conventional method which aims to prevent solidification of wax involves the addition of wax inhibition chemicals in the fluid stream, near the well head where the fluid is above the wax formation temperature.
Yet another method used is to maintain the fluid stream at a temperature above the wax formation temperature all the way from the well head to a production facility at surface or on land. This method typically involves electrical heating and highly efficient insulation of the pipeline.
In WO 2009051495 A1 Hoffmann et al disclose a method for wax removal and measurement of wax thickness. In a first step, Hoffmann et al suggest cooling the hydrocarbon fluid stream to a temperature below the wax formation temperature, causing precipitation of wax on the pipeline inner wall. In a second step, the pipeline is heated for a limited time period, sufficiently to cause release of the deposited wax from the pipeline wall. The wax is this way dislodged and returned to the fluid stream as solid particles, for further transport at a temperature below the wax formation temperature. To implement the method, Hoffmann et al suggest using a pipe-in-pipe flow system providing an annulus which is flooded with cold water for cooling the inner pipe and initiate precipitation of wax on the wall of the inner pipe. To release the wax the inner pipe may be heated by flooding the annulus with hot water, or the inner pipe may be heated by means of electrical heating using heating cables around the pipe, resistive or inductive heating in the pipe wall.
However, the Hoffmann et al document is not very detailed in its description of the structure and layout of the heated pipe-in-pipe pipeline that is utilized in the cooling and heating process.
In U.S. Pat. No. 6,955,221 B2 Bursaux discloses a pipe-in-pipe flow system for active heating of hydrocarbon liquid that is transported from a subsea wellhead to an above-surface hydrocarbon processing facility. A pipe is placed coaxially within an outer carrier pipe and the annulus between them is filled with thermally insulating material. For purposes of heating the hydrocarbon fluid flowing through the inner pipe, hot water is passed along the annulus via a single pipe or via multiple pipes installed in the insulation-filled annulus, or along an inner annulus formed by a water pipe added concentrically around the hydrocarbon-transporting inner pipe.
However, although Bursaux is detailed with respect to the structural buildup of the pipe-in-pipe flow system, the document lacks guidance regarding installation of electrical heating facilities in the pipe-in-pipe structure.
Another pipe-in-pipe flow system is previously disclosed in the Japanese patent publication JP-A-2009299752. An outer pipe formed of an elastic resin material is extruded about an inner pipe, and a plurality of legs extended in radial directions from the outer pipe to the inner pipe determines the spatial relation between the pipes. Notably, as far as understood, the pipe system of JP-A-2009299752 has no heating facilities installed and is intended for other purpose than flow assurance in hydrocarbon production subsea.